In the past, once a computer was configured for its working environment, that configuration rarely or never changed. In today's dynamic computing environment, however, a computer may need to change its configuration frequently. When, for example, a mobile computer moves from one wireless network to another, it might change its network address to one compatible with the new network. Also, when a computer temporarily joins an ad hoc networking group, administrative and security policies may require the computer to change its configuration to one more acceptable to the ad hoc group. In a third example, some computers change their configuration, at least their network address, every time they access the Internet through an Internet Service Provider (ISP).
Protocols have been developed to support various aspects of dynamic configuration. As one example, DHCP, the Dynamic Host Configuration Protocol, provides, among other network configuration information, an IP (Internet Protocol) address to a requesting computer. DHCP uses a client-server model, where a client computer, needing an IP address, requests one of a DHCP server. The DHCP server, in some cases provided by an ISP, controls a pool of IP addresses. Upon receiving the client's request, the DHCP server performs one of three modes of address allocation. In the “automatic allocation” mode, the DHCP server chooses an unused IP address from its pool and permanently assigns it to the requesting client. In the “manual allocation” mode, a network administrator chooses the address. Most interestingly for dynamic configuration, in the “dynamic allocation” mode, the DHCP server assigns a currently unused IP address to the client, but this dynamic address is valid only for a limited period of time as set by the DHCP server or until the client explicitly gives up its use of the address. Regardless of the allocation mode used, the DHCP server responds to the client's request by informing the client of the IP address assigned to it along with, in the case of dynamic allocation, the time period of the assignment. If the DHCP server cannot provide an IP address (possibly because all of the addresses in its pool are currently in use), then the DHCP server informs the client of this fact, and the client must wait until later to access the network.
In any dynamic computing system, ease of configuration should be balanced against security concerns. Many an institution establishes a dynamic network to perform the internal (that is to say, the private) work of the institution, and the institution can be compromised if an unauthorized computer is allowed to dynamically configure itself and join the network. Useful as they are, dynamic configuration protocols, including DHCP, are in general weak in the area of security. By relying heavily on post-configuration processes (such as encryption-key-based authentication mechanisms) to secure their network, some configuration servers inadvertently allow in rogue clients. While the post-configuration schemes usually work as advertised to limit the access and the possible harm of rogue clients, still, some harm can be done by a rogue even before it is forced to submit to, and to presumably fail, a post-configuration protection scheme.